【化】 刚玉

Corundum \Co*run"dum\ (k[-o]*r[u^]n"d[u^]m), n.; pl. {Corundums}
(k[-o]*r[u^]n"d[u^]mz). [Also corindon.] [From Hind. kurand
corundum stone.] (Min.)
The mineral alumina ({Al2O3}), as found native in a
crystalline state. Transparent varieties are used as
gemstones, including {sapphire}, which is the fine blue
variety; the {oriental ruby}, or red sapphire; the {oriental
amethyst}, or purple sapphire; and {adamantine spar}, the
hair-brown variety. It is the hardest substance found native,
next to the diamond.
[1913 Webster +PJC]

Note: The name corundum is sometimes restricted to the
non-transparent or coarser kinds. {Emery} is a
dark-colored granular variety, usually admixed with
magnetic iron ore.